Secondary batteries are devices capable of storing energy in chemical form and of converting into electrical energy to generate electricity when needed. The secondary batteries are also referred to as rechargeable batteries because they can be recharged repeatedly. Common secondary batteries include lead accumulators, NiCd batteries, NiMH accumulators, Li-ion batteries, Li-ion polymer batteries, and the like. When compared with disposable primary batteries, not only are the secondary batteries more economically efficient, they are also more environmentally friendly.
Secondary batteries are currently used in applications requiring low electric power, for example, equipment to start vehicles, mobile devices, tools, uninterruptible power supplies, and the like. Recently, as the development of wireless communication technologies has been leading to the popularization of mobile devices and even to the mobilization of many kinds of conventional devices, the demand for secondary batteries has been dramatically increasing. Secondary batteries are also used in environmentally friendly next-generation vehicles such as hybrid vehicles and electric vehicles to reduce the costs and weight and to increase the service life of the vehicles.
Generally, secondary batteries have a cylindrical, prismatic, or pouch shape. This is associated with a fabrication process of the secondary batteries in which an electrode assembly composed of an anode, a cathode, and a separator is mounted in a cylindrical or prismatic metal casing or a pouch-shaped casing of an aluminum laminate sheet, and in which the casing is filled with electrolyte. Because a predetermined mounting space for the electrode assembly is necessary in this process, the cylindrical, prismatic or pouch shape of the secondary batteries is a limitation in developing various shapes of mobile devices. Accordingly, there is a need for secondary batteries of a new structure that are easily adaptable in shape. Also, in the case of an electrolyte used in such a secondary battery, a suitable material having superior ionic conductivity without the problem of leakage is required.
In conventional electrochemical devices using an electrochemical reaction, a liquid electrolyte, particularly, an ion-conductive electrolyte in the form of an organic solution obtained by dissolving a salt in a non-aqueous organic solvent has been widely used as an electrolyte. However, the use of the liquid electrolyte may cause degradation of an electrode material, volatility rise of an organic solvent, safety-related problems such as combustion due to an increase in the ambient temperature and in the temperature of a battery itself, leakage, and difficulty in the preparation of an electrochemical device in various forms. In order to overcome safety problems due to the use of such a liquid electrolyte, polymer electrolytes such as a gel polymer electrolyte and a solid polymer electrolyte have been proposed. Generally, the safety of an electrochemical device increases in the order of a liquid electrolyte<a gel polymer electrolyte<a solid polymer electrolyte, whereas the performance thereof decreases in the same order. It is known that batteries using the solid polymer electrolyte cannot be commercialized due to the poor performances thereof. Meanwhile, the gel electrolyte in comparison to the liquid electrolyte has low ionic conductivity, poor mechanical property, and may also leak.
In addition, with the recent development of mobile devices, there is a need for a cable-type secondary battery which can freely change in shape. In the case of the cable-type secondary battery, an electrolyte layer tends to be damaged due to the repeated use of the battery, from which a short circuit between electrodes may occur. In order to solve this problem, an electrolyte having good mechanical property is required. However, since the mechanical property of the electrolyte and the ionic conductivity thereof have an inconsistent relationship, there exists a problem in that if the mechanical property of the electrolyte increases, the ionic conductivity thereof may decrease.